The Crusaders had to sweet talk the New Zealand Rugby Union before signing Samoan international Jeremy Su'a.

Before adding halfback Su'a, who has previously been contracted to the Waratahs and Melbourne Rebels, to their 32-man squad the Crusaders had to convince the NZRU the World Cup player was superior to any other New Zealander in his position.

Canterbury's Mark Swanepoel, Tasman's Steve Alfeld and New Zealand under-20 player Jono Kitto, all from within the Crusaders catchment, were overlooked.

Su'a played for Tasman in the ITM Cup after initially consulting with former Samoa assistant coach Aussie McLean - now the All Blacks defence coach - about playing in New Zealand.

Having targeted Su'a as a back-up to incumbents Andy Ellis and Willi Heinz, Crusaders representatives had to convince a selection panel including Neil Sorensen, Don Tricker and Graham Mourie their argument was sound.

It is understood that the Crusaders presented a document outlining the halfback's attributes.

"It was the same sort of situation as when we had Kahn Fotuali'i (who plays for Samoa and is now in Europe) in our squad a few years ago," Crusaders coach Todd Blackadder said.

"We just had to prove he was the next best option for us in the country. He is experienced, can play expansively and adapt to a high-tempo game."

Because of Su'a's Pacific Island heritage there was no need for the franchise to also seek the approval of the NZRU board - unlike when Englishman James Haskell was recruited to the Highlanders this season.

Su'a, 23, is a former Australian age-group representative. He made two appearances at last year's World Cup as a replacement for former Crusader Fotuali'i and had previously been on the Waratahs books for four seasons but never made any Super Rugby appearances.

Having been a member of the Melbourne Rebels extended squad since the World Cup and having studied civil engineering in Sydney, he was expected to return to Australia before the Crusaders swooped.

Su'a is not the only foreign halfback to be signed by a New Zealand franchise.

The Highlanders have added Japanese player Fumiaki Tanaka to the squad as All Black Aaron Smith's understudy.